1. Field of the Invention
This invention relates to a toroidal type continuously variable transmission for use, for example, as an automatic transmission for an automobile. More particularly, it relates to improvements in a power roller and a thrust bearing for supporting the power roller.
2. Related Background Art
It has been studied to use a toroidal type continuously variable transmission as schematically shown in FIGS. 8 and 9 of the accompanying drawings as a transmission for an automobile. This toroidal type continuously variable transmission, as disclosed, for example, in Japanese Laid-Open Utility Model Application No. 62-71465, has an input side disc 2 supported concentrically with an input shaft 1, and an output side disc 4 fixed to the end portion of an output shaft 3 disposed coaxially with the input shaft 1. Trunnions 6, 6 rockable about pivot shafts 5, 5 transverse to the input shaft 1 and the output shaft 3 are provided inside a casing containing the toroidal type continuously variable transmission therein.
The trunnions 6, 6 have the pivot shafts 5, 5 provided on the outer sides of the opposite end portions thereof. Also, the base end portions of displacement shafts 7, 7 are supported on the central portions of the trunnions 6, 6 and the trunnions 6, 6 are rocked about the pivot shafts 5, 5 to thereby make the angles of inclination of the displacement shafts 7, 7 adjustable. Power rollers 8, 8 are rotatably supported around the displacement shafts 7, 7 supported on the trunnions 6, 6. The power rollers 8, 8 are held between the input side and output side discs 2 and 4. The inner sides 2a and 4a of the input side and output side discs 2 and 4 which are opposed to each other have their cross-sections each forming a concave surface generated by rotating an arc centered on a pivot shaft 5 about the input shaft 1 or the output shaft 3. The peripheral surfaces 8a, 8a of the power rollers 8, 8 are formed into spherical convex surfaces and bear against the inner sides 2a and 4a.
A loading cam type pressing device 9 is provided between the input shaft 1 and the input side disc 2, and the input side disc 2 is resiliently pressed toward the output side disc 4 by this pressing device 9. The pressing device 9 is comprised of a cam plate 10 rotatable with the input shaft 1, and a plurality of (e.g. four) rollers 12, 12 held by a holder 11. A cam surface 13 which is an uneven surface in a circumferential direction is formed on one side (the left side as viewed in FIGS. 8 and 9) of the cam plate 10, and a similar cam surface 14 is formed on the outer side (the right side as viewed in FIGS. 8 and 9) of the input side disc 2. The plurality of rollers 12, 12 are supported for rotation about radial axes relative to the center of the input shaft 1.
When during the use of the toroidal type continuously variable transmission constructed as described above, the cam plate 10 rotates with the rotation of the input shaft 1, the cam surface 13 presses the plurality of rollers 12, 12 against the cam surface 14 on the outer side of the input side disc 2. As the result, the input side disc 2 is pressed against the power rollers 8, 8 and at the same time, the input side disc 2 is rotated on the basis of the pressing of the pair of cam surfaces 13, 14 against the plurality of rollers 12, 12. This rotation of the input side disc 2 is transmitted to the output side disc 4 through the power rollers 8, 8 to thereby rotate the output shaft 3 fixed to the output side disc 4.
When the rotational speed ratio (the transmission gear ratio) between the input shaft 1 and the output shaft 3 is to be changed and deceleration is first to be done between the input shaft 1 and the output shaft 3, the trunnions 6, 6 are rocked about the pivot shafts 5, 5 and the displacement shafts 7, 7 are inclined so that as shown in FIG. 8, the peripheral surfaces 8a, 8a of the power rollers 8, 8 may bear against the portion toward the center of the inner side 2a of the input side disc 2 and the portion toward the outer periphery of the inner side 4a of the output side disc 4, respectively. When conversely, acceleration is to be done, the displacement shafts 7, 7 are inclined so that as shown in FIG. 9, the peripheral surfaces 8a, 8a of the power rollers 8, 8 may bear against the portion toward the outer periphery of the inner side 2a of the input side disc 2 and the portion toward the center of the inner side 4a of the output side disc 4, respectively. If the angles of inclination of the displacement shafts 7, 7 are made medium between FIGS. 8 and 9, a medium transmission gear ratio will be obtained between the input shaft 1 and the output shaft 3.
Further, FIGS. 10 and 11 of the accompanying drawings show a toroidal type continuously variable transmission described in the microfilm of Japanese Utility Model Application No. 63-69293 (Japanese Laid-Open Utility Model Application No. 1-173552). An input side disc 2 and an output side disc 4 are rotatably supported around an input shaft 15 needle bearings 16 and 16, respectively. Also, a cam plate 10 is spline-engaged with the outer peripheral surface of an end portion (the left end portion as viewed in FIG. 10) of the input shaft 15, and is prevented from moving away from the input side disc 2 by a flange portion 17. The cam plate 10 and rollers 12, 12 together constitute a loading cam type pressing device 9 for rotating the input side disc 2 while pressing it toward the output side disc 4 on the basis of the rotation of the input shaft 15. An output gear 18 is coupled to the output side disc 4 by keys 19, 19 so that the output side disc 4 and the output gear 18 may rotate in synchronism with each other. The output gear 18 and a gear or the like, not shown, meshing with the output gear 18 together constitute a power output mechanism to output the rotation of the output disc.
Pivot shafts 5, 5 provided at the opposite end portions of a pair of trunnions 6, 6 are supported on a pair of yokes 20, 20 for rocking movement and displacement in an axial direction (the front to back direction as viewed in FIG. 10 or the left to right direction as viewed in FIG. 11). The pair of yokes 20, 20 are in the form of metallic plates having sufficient rigidity, and have circular holes formed in the central portions thereof fitted on support posts 24a, 24b secured to the inner surface of a casing 22 or the side of a cylinder case 23 provided in the casing 22, whereby they are supported for rocking movement and displacement in the axial direction of the pivot shafts 5, 5. Circular support holes 25, 25 are formed in the opposite end portions of the yokes 20, 20, and the pivot shafts 5, 5 are supported in the support holes 25, 25 by radial needle bearings 27, 27 provided with outer races 26, 26. On the basis of these constructions, the trunnions 6, 6 are supported in the casing 22 for rocking movement about the pivot shafts and displacement in the axial direction of the pivot shafts 5, 5.
Displacement shafts 7, 7 are supported in circular holes 52, 52 formed in the intermediate portions of the trunnions 6, 6 supported in the casing 22 in the manner described above. These displacement shafts 7, 7 have support shaft portions 28, 28 and pivot shaft portions 29, 29 parallel to each other and eccentric with respect to each other. The support shaft portions 28, 28 are rockably supported inside the circular holes 52, 52 through radial needle bearings 30, 30. Power rollers 8, 8 are rotatably supported around the pivot shaft portions 29, 29 through radial anti-friction bearings such as radial needle bearings 31, 31.
The pair of displacement shafts 7, 7 are provided at opposite positions of 180.degree. about the input shaft 15. Also, the directions in which the pivot shaft portions 29, 29 of the displacement shafts 7, 7 are eccentric with respect to the support shaft portions 28, 28 are the same direction (the right to left direction as viewed in FIG. 11) with respect to the direction of rotation of the input side and output side disc 2 and 4. Also, the direction of eccentricity is a direction substantially orthogonal to the direction of disposition (the left to right direction as viewed in FIG. 10 or the front to back direction as viewed in FIG. 11) of the input shaft 15. Accordingly, the power rollers 8, 8 are supported for some displacement in the direction of disposition of the input shaft 15. As the result, even if due to the irregularity of the dimensional accuracy of any constituent part or the resilient deformation or the like during the transmission of power, the power rollers 8, 8 tend to be displaced in the axial direction (the left to right direction as viewed in FIG. 10 or the front to back direction as viewed in FIG. 11) of the input shaft 15, this displacement can be absorbed without any unreasonable force being applied to each constituent part.
Also, between the outer sides of the power rollers 8, 8 and the inner sides of the intermediate portions of the trunnions 6, 6, thrust anti-friction bearings such as thrust ball bearings 32, 32 and thrust bearings such as thrust needle bearings 34, 34 supporting a thrust load applied to outer races 33, 33 which will be described next are provided in succession from the outer sides of the power rollers 8, 8. The thrust ball bearings 32, 32 permit the rotation of the power rollers 8, 8 while supporting a load in a thrust direction applied to the power rollers 8, 8. Also, the thrust needle bearings 34, 34 permit the pivot shaft portions 29, 29 and the outer races 33, 33 to rock about the support shaft portions 28, 28 while supporting a thrust load applied from the power rollers 8, 8 to the outer races 33, 33 of the thrust ball bearings 32, 32.
Also, driving rods 35, 35 are coupled to one end portion (the left end portion as viewed in FIG. 11) of the trunnions 6, 6 and driving pistons 36, 36 are secured to the outer peripheral surfaces of the intermediate portions of the driving rods 35, 35. The driving pistons 36, 36 are oil-tightly fitted in driving cylinders 37, 37 provided in the cylinder case 23. Further, a pair of anti-friction bearings 39, 39 are provided between a support wall 38 provided in the casing 22 and the input shaft 15 to thereby rotatably support the input shaft 15 in the casing 22.
In the case of the toroidal type continuously variable transmission constructed as described above, the rotation of the input shaft 15 is transmitted to the input side disc 2 through the pressing device 9. The rotation of this input side disc 2 in turn is transmitted to the output side disc 4 through the pair of power rollers 8, 8 and further, the rotation of the output side disc 4 is output via the output gear 18. When the rotational speed ratio between the input shaft 15 and the output gear 18 is to be changed, the pair of driving pistons 36, 36 are displaced in opposite directions. With the displacement of the driving pistons 36, 36, the pair of trunnions 6, 6 are displaced in opposite directions, and for example, the lower power roller 8 in FIG. 11 is displaced to the right as viewed in FIG. 11 and the upper power roller 8 in FIG. 11 is displaced to the left as viewed in FIG. 11. As the result, the direction of a force in the tangential direction acting on the portions of contact between the peripheral surfaces 8a, 8a of the power rollers 8, 8 and the inner sides 2a and 4a of the input side disc 2 and the output side disc 4, respectively, changes. With the change in the direction of this force, the trunnions 6, 6 rock in opposite directions in FIG. 10 about the pivot shafts 5, 5 pivotally supported on the yokes 20, 20. As the result, as shown in FIGS. 8 and 9, the positions of contact between the peripheral surfaces 8a, 8a of the power rollers 8, 8 and the above-mentioned inner sides 2a, 4a change, and the rotational speed ratio between the input shaft 15 and the output gear 18 changes.
When the power rollers 8, 8 are displaced axially of the input shaft 15, the displacement shafts 7, 7 pivotally supporting the power rollers 8, 8 slightly rock about the support shaft portions 28, 28. As the result of this rocking movement, the outer sides of the outer races 33, 33 of the thrust ball bearings 32, 32 and the inner sides of the trunnions 6, 6 are displaced relative to each other. The thrust needle bearings 34, 34 are present between these outer sides and inner sides and therefore, the force required for this relative displacement is small. Accordingly, the force for varying the angles of inclination of the displacement shafts 7, 7 as described above may be small.
In the case of the toroidal type continuously variable transmission constructed and acting as described above, when the torque to be transmitted becomes excessively great, there is the possibility that the durability of the outer races 33, 33 of the thrust ball bearings 32, 32 cannot be secured on the basis of the resilient deformation of the trunnions 6, 6. During the operation of the toroidal type continuously variable transmission, a great thrust load is applied from the power rollers 8, 8 supported on the trunnions 6, 6 to the inner sides (the surfaces opposed to the input shaft 15) of the intermediate portions of the trunnions 6, 6. On the basis of this thrust load, the trunnions 6, 6 become curved in such a direction that the inner sides thereof become concave surfaces, as exaggeratedly shown in FIG. 12 of the accompanying drawings.
When such curvature occurs, the pressure of contact among the rolling surfaces of a plurality of balls 40, 40 of the thrust ball bearings 32, 32 and outer race tracks 41, 41 formed on the inner sides of the outer races 33, 33 and inner race tracks 42, 42 formed on the outer sides of the power rollers 8, 8 becomes non-uniform. Specifically, the pressure of contact among the rolling surfaces of the balls 40, 40 present on the portions toward the lengthwisely opposite ends of the trunnions 6, 6, i.e., the left and right side portions as viewed in FIGS. 11 and 12, and the outer race tracks 41, 41 and the inner race tracks 42, 42 becomes great. Conversely, the pressure of contact among the rolling surfaces of the balls 40, 40 present on the portion toward the widthwisely opposite ends of the trunnions 6, 6, i.e., the left and right side portions as viewed in FIG. 10, and the outer race tracks 41, 41 and the inner race tracks 42, 42 becomes small.
The power rollers 8, 8 and the balls 40, 40 rotate or revolve with the operation of the toroidal type continuously variable transmission and therefore, the above-mentioned rolling surfaces and the inner race tracks 42, 42 are substantially evenly subjected to a load. Accordingly, it never happens that the fatigue of these rolling surfaces and inner race tracks 42, 42 locally progresses. In contrast, the outer races 33, 33 are fitted and fixed to the continuing portion between the support shaft portion 28 and the displacement shaft portion 29 in the intermediate portion of the displacement shaft 7 and do not rotate. Therefore, great pressure of contact is always applied to the same portions (the portions near the left and right ends as viewed in FIGS. 11 and 12) of the outer race tracks 41, 41, and the fatigue of these portions locally progresses. As the result, the service life of the thrust ball bearings 32, 32 become short, and this is not preferable in securing the durability of the toroidal type continuously variable transmission.
Also, the basic construction itself of the power rollers 8, 8 and the thrust ball bearings 32, 32 incorporated in the toroidal type continuously variable transmission constructed and acting as described above can be considered to be thrust ball bearings having the power rollers 8, 8 as inner races and supporting a thrust load applied between the power rollers 8, 8 and for the outer races 33, 33 by the balls 40, 40. However, the power rollers 8, 8 and the thrust ball bearings 32, 32 incorporated in the toroidal type continuously variable transmission it is difficult to secure the durability thereof from the speciality of the form of use thereof, as compared with ordinary thrust ball bearings. The reasons for this are as stated in the following items (1) to (3).
(1) Bending stress applied to the power rollers 8, 8 and the outer races 33, 33 becomes very great. In contrast, ordinary thrust anti-friction bearings are of such structure that, for example, ten or so rolling members such as balls are evenly subjected to a thrust load, and bending stress is hardly applied to the inner and outer races therefore, the strength of the inner and outer races against the bending stress is not very important.
The peripheral surfaces 8a, 8a of the power rollers 8, 8 incorporated in the toroidal type continuously variable transmission strongly bear against the inner sides 2a and 4a of the input side disc 2 and the output side disc 4, respectively, at two circumferentially opposite locations. Therefore, a thrust load applied from these two discs 2 and 4 to the power rollers 8, 8 becomes non-uniform in the circumferential direction thereof, and great bending stress is applied to the power rollers 8, 8 and the outer races 33, 33 receiving a thrust load from these power rollers 8, 8 through the balls 40, 40.
The bending stress applied to the power rollers 8, 8 and the outer races 33, 33 in this manner differs greatly depending on the operative situation of the toroidal type continuously variable transmission, the transmitted torque, the thicknesses of the members 8 and 33, etc., and reaches the order of 100 kgf/mm.sup.2 at greatest. If no countermeasure is provided for the power rollers 8, 8 and the outer races 33, 33 to which such great bending stress is applied, these members 8 and 33 will become apt to be damaged early and sufficient durability cannot be secured.
(2) In the case of the half-toroidal type continuously variable transmission as shown in FIGS. 8 to 11, the pressure of contact applied to the portions of contact among the rolling surfaces of the balls 40, 40 and the inner race track and the outer race track becomes very high. In the case of ordinary thrust anti-friction bearings, the pressure of contact P.sub.max on the portion of contact between the rolling surface of each rolling member and the track surface of each race is of the order of 2-3 GPa. In contrast, in the case of the thrust ball bearings 32, 32 incorporated in the toroidal type continuously variable transmission, the pressure of contact P.sub.max on the portions of contact among the rolling surfaces of the balls 40, 40 and the inner race track and the outer race track reaches the order of 2.5-3.5 GPa. Particularly, during the maximum deceleration as shown in FIG. 8, the pressure of contact P.sub.max may reach the order of 4 GPa.
When the pressure of contact P.sub.max becomes so great, the diameter of a contact ellipse present in the portion of contact among the rolling surfaces of the balls 40, 40 and the inner race track and the outer race track becomes large. For example, in the case of an ordinary thrust ball bearing having an outer diameter of 200 mm or less, the minor diameter of the contact ellipse is less than 1 mm, whereas in the case of the thrust ball bearings 32, 32 incorporated in the toroidal type continuously variable transmission, the minor diameter reaches the order of 1.5 mm. When the diameter of the contact ellipse thus becomes large, the depth a maximum shearing stress reaches becomes great, and unless the thickness of a hardened layer formed on the surface portions of the inner race track and outer race track is made great, the service life of these track portions cannot be secured.
(3) The portions of contact between the inner sides 2a and 4a of the input side and output side discs 2 and 4, respectively, and the peripheral surfaces 8a, 8a of the power rollers 8, 8 effect the transmission of very great power while spinning. In the case of the toroidal type continuously variable transmission which is a traction drive transmission, a contact ellipse portion having a diameter of only several millimeters present in the above-described portions of contact transmits as great power as the order of 50 kw. Moreover, spin takes place in this contact ellipse portion and therefore, great shearing stress and heat generation occur to this contact ellipse portion. Therefore, unless the strength of the above-mentioned peripheral surfaces 8a, 8a is made high, the durability of the power rollers 8, 8 cannot be sufficiently secured.